In liquid-liquid optofluidic devices, light can be manipulated purely by liquid flow, thus eliminating the need for mechanical or electrical light-manipulating mechanisms. Various tunable optofluidic microlenses based on liquid-liquid interfaces have been successfully demonstrated. The key to these liquid-liquid interface-based microlenses is to create a hydrodynamically tunable, curved liquid-liquid interface between two side-by-side flowing fluids having different refractive indices. The tunable liquid-liquid interface can then act as a refractive lens to variably focus light.
We developed a lens that is sized and configured as a microlens, as shown in FIG. 1. The microlens is configured to operate such that the lens has an optically smooth, curved fluidic interface. A refractive contrast between fluids is used to focus light emitted from a light source, such as an optical fiber. For instance, two fluids with different refractive indices may be used, such as a calcium chloride (CaCl2) solution and distilled water. The two fluids can be coinjected into a microfluidic device that has a 90 degree curve. The fluids may flow along the curved channel in a laminar flow to provide an optically smooth, nearly planar or substantially planar interface between the co-injected fluids. Upon entering the curved portion of the channel, the fluids are perturbed by a Dean flow effect due to a centrifugal force induced within the curved portion of the channel, which causes the originally flat fluidic interface to bow outwardly and form a cylindrical microlens. The curvature of the fluidic interface and, therefore, the focus point of the fluidic lens can also be conveniently adjusted by simply changing the flow rates of one or both fluids.
The embodiment of our lens shown in FIG. 1 has an input light that is guided to the lens by an optical fiber connected to a light source. The optical fiber is located near the exit, or output end, of the 90 degree curve where the cylindrical lens is formed. The image of the focused light can be observed from the other side of the channel.
The behavior of the calcium chloride solution and distilled water were studied using computational fluid dynamic (“CFD”) simulation and the resulted fluidic interface profiles were used for optic ray tracing simulation, which is shown below in FIGS. 2A-2C. The simulation results showed that the curved fluidic interface can be used to focus the light and that the lens profile and focal length can be tuned by changing the flow rate of the water, the calcium chloride solution, or both fluids in the laminar flow of fluid in the channel. The mechanism by which the embodiment of the optofluidic cylindrical lens operates is also illustrated in FIGS. 2D and 2E.
FIGS. 3A-3C illustrate the direct visualization of focused light at different flow rates that may be provided by the lens shown in FIG. 1. While this device is able to adjustably focus light, it is unable to adjust the angle at which light is reflected by a lens. Instead, the light is only able to be reflected in the direction the light is emitted. Further, the above mentioned lens shown in FIG. 1 has structural limitations that may be detrimental to fabrication and use of the lens. For instance, a light emitting device is configured for passing light through a portion of fluid passing through a straight portion of the channel and further requires the channel to be curved. Such structural limitations are undesirable and are design constraints. Such a lens usually must be configured to have fluid flowing in a direction that is perpendicular to the light passing through the channel. Further, the curved path defined by the channel was believed to be necessary to perturb the fluids via a Dean flow effect to cause the flat fluidic interface to bow outwardly and form a cylindrical microlens for reflecting light from a light emitting device.
Typically, liquid-liquid-interface-based microlens designs have two major shortcomings. Firstly, diffusion of solute across the fluid-fluid interface tends to smear the fluid-fluid interface, and therefore, a high flow rate is required to minimize the diffusion time and maintain a relatively clear liquid-liquid interface. Secondly, most of the current liquid-liquid-interface-based microlenses are only capable of line focusing (1D focusing), instead of point focusing (2D focusing). This is because current microfluidic technology has not yet been able to provide a means to conveniently create the precisely controlled, 3D liquid-liquid interface needed for 2D focusing.
A new lens device is needed that can provide a precisely controllable 3D liquid-liquid interface that may be used for 2D focusing of light. Such a lens device is also preferably configured to avoid smearing at any liquid-liquid interface. Such a lens device may additionally be preferably developed such that a curved channel is not necessary so that a wide variety of design configurations may be utilized for embodiments of such a lens device.